Solid lasers (using mostly rare earth-doped crystals or glasses, e.g., Nd:YAG, Nd:YV0.sub.4, Nd:YAlO, Nd:YLF, Nd:glass or other, similar solid materials) with frequency doubling within the cavity have been known for a long time and are used for many applications in laser technology. What is used here is the generation of the second or higher harmonic vibrations in materials (mostly crystals, which have no inversion center, e.g., KTP, LBO, BBO, KNbO.sub.3, LiNbO.sub.3 or other) with a high nonlinear coefficient, which generates light of double (or multiple) frequency of the irradiated light wave by anharmonic vibrations of the lattice atoms, excited by an incident light wave. The process of generating higher harmonics strongly depends on the power density (cf., e.g., Kochner, Solid-State Laser Engineering), so that to generate frequency-doubled laser radiation of high efficiency, the nonlinear crystal is often introduced (at least in the case of continuously working (cw) lasers) either into the cavity of the laser itself or into a separate cavity (see above or, e.g., Yariv, Quantum Electronics, 3rd ed., p. 402). Even though the latter case of a separate cavity for the frequency doubler offers the fundamental advantage of low amplitude variations, this arrangement is characterized by a considerable complication due to the fact that the cavity belonging to the frequency doubler crystal must be actively stabilized to the frequency of the laser cavity and the laser radiation should be possibly a single-frequency radiation to achieve a high efficiency. The first case of introducing the frequency doubler crystal into the laser cavity is substantially less complicated compared with this; it is possible to work in this case with lasers which emit in longitudinal modes ranging in number from a few to many; the cavity mirrors are usually selected to be highly reflective mirrors for the laser wavelength in order to achieve a maximum increase in power in the cavity and thus the highest possible efficiency of doubling; at the same time, the output mirror is highly transmittent for the frequency-doubled radiation in order to be able to properly decouple it from the cavity.
However, this arrangement has a loud amplitude noise, which is an inherent feature of the system and which was first described, to the best of our knowledge, by T. Baer in J. Opt. Soc. Am. B, Vol. 3, No. 9, September 1986, p. 1175. There are many different approaches to explain this noise. Baer explains this by a competition of different modes (since the actually most intense mode is doubled best, it is attenuated most by the decoupling from the laser cavity, and another longitudinal mode will now become the most intense one, etc.). Other explanations are based on the total frequency generation or on the competition between modes of different polarization (cf., e.g., EP 0 457 590 A2). However, all these mechanisms are probably involved in the noise process at the same time.
The fact that the laser may have a very low-frequency noise, which is manifested by a "flickering" of the laser beam, whose degree of modulation may reach up to 100%, is especially disturbing for many applications. This noise is highly chaotic because of the nonlinear relationship of the doubling efficiency (see Koechner, see above); stable states may become temporarily established, which may be abruptly followed by loud noise. This phenomenon has been investigated in the literature in detail (see, e.g., Phys. Rev. A, Vol. 41, No. 5, March 1990, p. 2778, or Opt. Comm., Vol. 118 (1995), p. 289). Preliminary control models which are to eliminate this chaotic noise have also been designed, but the control bandwidth of these controllers is currently too narrow by several orders of magnitude (bandwidths markedly exceeding 1 MHz were necessary for a nonlinear control loop) in order to practically suppress the noise (see, e.g., Phys. Rev. A, Vol. 47, No. 4, April 1993, p. 3276).
Other approaches to minimizing the noise are based according to the state of the art on the introduction of a quarter-wave plate (see U.S. Pat. No. 4,618,957) or a Brewster plate (see DE 3 917 902 A1) into the cavity or on the temperature stabilization of the doubler crystal (see EP 0 329 442 A2). However, all these solutions according to the state of the art have substantial drawbacks.
Even though the introduction of an additional element into the cavity (quarter-wave plate or Brewster plate) makes it possible to extensively suppress the flickering of the laser, such elements in the cavity are to be adjusted very accurately, which increases the expense of manufacture, and these elements always lead to higher losses in the cavity (because of residual reflections and scattering) even in the case of the best adjustment, so that these higher losses also drastically reduce the power density and consequently the efficiency of doubling.
In contrast, the stabilization of the temperature of the doubler crystal (EP 0 329 442 A2) can be accomplished without such additional elements and it also makes it possible to markedly reduce the laser noise at equal power density. The reason for this might be that in the case of the angle-dependent phase matching (and such phase matching is involved here, cf. Koechner, p. 528), the doubler crystal itself acts as a quarter-wave plate of a very high order, which has the same effect as an additionally introduced quarter-wave plate, but without having to be additionally adjusted or without offering additional reflection or scattering surfaces. However, the exact adjustment of the length of the doubler crystal to an integer multiple of .lambda./4 takes place by an exact adjustment of the length of the doubler crystal via the temperature.
This process already comes very close to the process according to the present invention, but it still has an essential shortcoming. Since the laser cavity changes its temperature in the course of the operation, especially during changes in the environmental conditions or if a thermal equilibrium is reached only incompletely, there will be continuous changes in the length of the laser cavity and also in the exact temperature of the doubler crystal. Therefore, a pure stabilization of the crystal length to the temperature permits only operation in a very narrow temperature window under definitely constant environmental conditions and at the stable, steady thermal equilibrium (see EP 0 329 442 A2). However, these conditions are not usually satisfied in a laser operated under the real conditions of an application.